Carbon nanotubes (CNT), including multiwall carbon nanotubes (MWNT), have been incorporated into various polymer systems owing to their outstanding tensile modulus (270-950 GPa), tensile strength (11-63 GPa), as well as thermal (200-3000 W/m/K at 300 K) and electrical conductivity (102-107 S/m at 300 K). While interfacial stress transfer from nanotubes to polymer accounts for mechanical reinforcement, CNT can also act as a nucleating agent for polymer crystallization, as well as induce the formation of a highly ordered interphase polymer layer in many polymer/CNT composites.
Polypropylene (PP) is a widely used commercial polymer due to its excellent chemical stability, physical and mechanical properties, processability and low cost. PP has numerous industrial applications, such as fiber manufacturing, packaging, automotive components, construction, etc. Over the years, PP has been reinforced with fibrous fillers (e.g., carbon, Kevlar, natural fibers) and particulate fillers (e.g., talc, mica, clay), as well as by melt blending with other polymers to enhance its mechanical properties. Emergence of CNT has paved the way for new polymer composite applications in various fields, such as electromagnetic interference (EMI) shielding, enhanced oxygen barrier performance, and for antistatic purposes.
Although the benefits of introducing CNT into PP can be many fold, it is difficult to fully disperse CNT into the PP polymer matrix, since CNT tend to bundle together through van der Waals forces. Only a limited number of organic solvents, such as N,N-dimethylformamide (DMF), 1,2-dichlorobenzene (ODCB), N-methylpyrrolidinone (NMP), tetrahydrofuran (THF) and chloroform, are known to disperse CNT to some extent. Nanocomposites incorporating CNT can be successfully produced via a solution approach with conjugated polymers, aromatic polymers, and other polar polymers like pol(vinyl chloride), polyacrylonitrile, poly(methyl methacrylate), poly(vinyl alcohol) and poly (ethylene oxide), etc. However, PP, due to its non-polar nature, does not have sufficient solubility in these solvents.
Several approaches have been used for improving the dispersion of CNT in PP, for example chemical modification of CNT and/or the polymer through covalent functionalization and grafting; incorporation of a compatibilizer such as maleic anhydride grafted PP (MA-g-PP) and maleic anhydride grafted styrene-ethylene/butylenes-styrene copolymer (MA-SEBS); surfactants, such as sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (NaDDBS); master batch dilution; mechanical dispersion including ultra-sound assisted melt extrusion; and some combinations thereof.
PP coating of CNT has been reported via grafting of MA-g-PP or PP. One method stabilizes CNT in aqueous solution via non-covalent association between nanotubes and poly(vinyl pyrrolidone) (PVP), wherein the PVP wrapped CNT can be further dispersed in other polymers, e.g., in poly(vinyl alcohol). Another method produced PE wrapped CNT via crystallization in ODCB, wherein the MWNT were wrapped by a homogenous coating of PE, while a PP coating could not be achieved by the same procedure. In yet another method, CNT were encapsulated with MA-g-PP via a solution mixing approach by using butanol and xylene. Ball milling was also used for forming CNT polymer composites, and while some MA-g-PP chains adsorbed onto sidewalls of MWNT, PP did not exhibit the same behavior. Thus, there is an ongoing need for the development of PP coated CNT (PP/CNT) and methods of making same, wherein the PP encapsulates the CNT via a non-perturbing PP coating (e.g., a PP coating via non-covalent means) to prevent nanotube re-aggregation.